1. Field of the Invention
The present invention relates to a thermal printer and a driving method of a thermal head of the thermal printer. More particularly, the present invention relates to a thermal printer which detects the temperature of the thermal head itself and the ambient temperature of the thermal head in order to control temperature variation of the thermal head.
2. Background Art
The thermal printer has a thermal head having a plurality of heating elements which are connected in parallel to one another and arranged in an array. The thermal head gives an amount of heat energy to the thermosensitive recording medium depending on the sensitivity of the recording medium and desirable density of the pixel to record. Specifically, first bias heat energy is applied for heating the thermosensitive recording medium up to such a temperature above which a color begins to be developed. Next, a variable amount of image heat energy necessary for developing the color at desirable density is applied.
On the other hand, the thermal head has an array of heating elements, each constituted of a resistance. Generally, a resistance layer is formed on a substrate, and a pair of electrode layers overlap the opposite ends of the resistance layer. A grazed glass layer is interposed between the resistance layer and the substrate so as to make the outer surface of the heating element convex, and thus ensure the contact with the recording medium or the ink ribbon. On the inner surface of the substrate, i.e., the opposite side from the grazed glass layer, a heat radiation plate is mounted for rapidly cooling the heating element in the cooling periods provided between the heating periods of the heating elements.
The heating elements have a problem that as the print proceed, the thermal head stores heat energy especially in the grazed glass layers, and hence the temperature of the thermal head gradually rises in total. Even though a predetermined electric driving energy is supplied to the resistance to generate a predetermined heat energy, the heat energy applied to the recording medium may change depending upon the temperature of the heating element and the thermal head. Especially the heat accumulation in the grazed glass layer has great influence on the surface temperature of the heating element. As a result, density of an image recorded on a sheet of recording paper tends to be low in the first stage of the printing, and relatively high in the end of the printing. Also when printing a plurality of copies in continuous succession, the first copy tends to have a lower density in total, while the last copy tends to have a higher density in total. This phenomenon is called "shading".
Hereinafter, the variation of temperature of the thermal head due to heat storage or accumulation during printing will be referred to as "a long interval temperature variation of the thermal head".
To solve this problem, JPB 60-240271 discloses a thermal printer wherein drive voltage supplied to the thermal head is controlled based on a temperature measured from the thermal head, such that the head drive voltage is lowered with increasing head temperature, thereby to minimize unexpected variation of the heat energy radiated from the thermal head, i.e. the shading.
Because not only the head temperature but also the temperature of the ink ribbon and/or the recording medium and that of the platen drum or plate have influence on the recording density, it is desirable to take the ambient temperature into consideration. JPB 2-162060 discloses a teaching to provide a second temperature sensor in the ambience of the thermal head, so as to control the energy supply to the thermal head on the basis of the temperature signals from the two sensors.
However, considering the structure of the heating elements as above, it is hard to dispose a temperature sensor, such as a thermistor, adjacent to the outer surface of the heating element or the grazed glass layer. The head temperature sensor is usually disposed in the substrate which is constituted of a ceramic plate and/or an aluminum plate, so that the substrate is interposed between the temperature sensor and the grazed glass layer.
Accordingly, the temperature detected by the conventional head temperature sensor does not directly represent the temperature of the grazed glass layer, but represent the temperature of the substrate which has influence merely indirectly on the surface temperature of the heating element. For this reason, the accuracy of the temperature control of the heating elements and thus the preventing effect against the shading has been insufficient.